Amino acid side chains play fundamental roles in stabilising protein structures and in catalysing enzymatic reactions. These fields are increasingly investigated by infrared spectroscopy at the molecular level. To help the interpretation of the spectra, a review of the infrared absorption of amino acid side chains in H2O and (H2O)-H-2 is given. The spectral region of 2600-900 cm(-1) is covered. (C) 2001 Elsevier Science Ltd. All rights reserved.